The specific aims of this renewal application remain the determination of the bioactive conformations required at both mu and delta opioid receptors. This determination will include the identification of the pharmacophoric groups of the ligand, i.e., those moieties which provide energetically favorable interactions with complementary sites of the receptor, as well as the elucidation of the relative geometry between these groups. Of particular interest will be structural and conformational differences which distinguish mu from delta ligands. Our approach employs both experimental (primarily NMR) and theoretical techniques to develop models of these bioactive conformations both upon information garnered from conformationally constrained, receptor selective ligands int he absence of receptor. Since even the conformationally constrained, cyclic peptides studied here contain residual flexibility, particularly in the pharmacophore elements, several low energy conformations will in general be accessible, any one of which may be the sought after bioactive conformer. Identification of this bioactive conformation, then, requires comparison among a set of related peptides in which further conformational constraints are incorporated into the flexible elements of the parent peptide. Correlation of opioid binding affinity of this set of analogs with the energetically accessible regions of conformational space available to each allows a more realistic elaboration of the bioactive conformation. The recent cloning and sequence elucidation of mu and delta receptor and the recognition that they share the common structural motif of the G-protein coupled receptor superfamily raises the hope that the molecular details of the ligand- receptor interaction may be accessible. We propose plans to develop structural models of the mu and delta receptors and to use the complementary conformational and pharmacological data from our series of analogs to unravel details of these ligand-receptor interactions.